For online sellers the key to make or break is finding those customers. rather than limiting yourself to just eBay and Amazon there are lots of alternative market places out there. UK is a small island with limited number of customers but if you think world wide then your world becomes your oyster.
There are a number of other places to trade besides eBay and Amazon that harness the European market and North American Market adding millions of potential customers to your wish list including those corners of the UK Market that go largely ignored.
Besides the usual Facebook Marketplaces, Linnworks have put together a comprehensive guide to other Market places which is worth a peek at and gives you links to sites such as Fruugo, Flubit and Allegro. (find that here)
If you are thinking more on the used side of things then places like Pre-loved, Depop, eBid, Gumtree, Etsy, ASOS, Vinted, Shpock.
There are also lots of online auction houses you can list your items.
There is a whole world out there outside of eBay just waiting all you need is time to get to know what will work best for the items that you sell, there are a whole host more of selling places online that I have not even covered here that are specialist sites such as those that deal in handmade goods and antiques and collectables and I also forget to mention your own webstore.